1. Field of the Invention
The present invention relates to a video processing technique, and more specifically to a technique for effectively preventing image quality degradation, such as burn-in, in a liquid crystal display device.
2. Description of the Related Art
Liquid crystal display devices (such as direct-view-type liquid crystal display devices, and liquid crystal projectors) employ a method to adjust the amount of light constantly emitted from a light source by using a liquid crystal shutter, and are, therefore, referred to as hold-type display devices. Hold-type display devices emit light on a one-frame-period basis; therefore, by conducting visual following (viewing an image performed by following moving portions of an image with the eyes in moving image display), “motion blur” corresponding to light emission periods is observed. As a method for solving this problem, a pseudo-impulse driving method is used, for example. In the pseudo-impulse driving method, sub-frames are generated by multiplying a frame frequency N-fold. Thereafter, spatial high-frequency components greatly contributing to “motion blur” are concentrated on one of the sub-frames and thus a high-frequency enhance image is displayed; on the other hand, in other sub-frames, a low-frequency image is displayed in which spatial low-frequency components are distributed, (see Japanese Patent Laid-Open No. 2002-351382, for example).
In general, when a direct voltage has been applied between the pixel electrode and the counter electrode of liquid crystal display devices for hours, an imbalance in the ion density is created inside, and thus their tone characteristics cannot be reproduced, that is, so-called “burn-in” occurs. Because of this, to prevent the creation of the imbalance in the ion density in the liquid crystal display device, driving is performed in which the polarity of a video signal voltage applied to the liquid crystal sub-frame by sub-frame is periodically inverted relative to a common electrode voltage (VCOM).
As a result, the problem has arisen that since there is a difference in the applied voltage between sub-frames in such pseudo-impulse driving, an imbalance in the polarity of the applied voltage occurs, and thus burn-in occurs during their long-time use. As a method to solve such a problem, Japanese Patent Laid-Open No. 2008-064919 discloses a technique of preventing an imbalance in the polarity of applied voltages by extending a polarity inversion driving period from a sub-frame-by-sub-frame basis to a frame-by-frame basis.
However, in the method of extending the polarity inversion period disclosed in Japanese Patent Laid-Open No. 2008-064919, there is a problem that since the polarity inversion period increases, flicker tends to occur.